Re: Really strange performance issue

  • From: Andrew Kerber <andrew.kerber@xxxxxxxxx>
  • To: Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>
  • Date: Fri, 31 Oct 2014 07:37:11 -0500

Jonathan - you are correct on the oddities, but there is no unique constraint.  
The row is unique however, and there are only about 30 rows in the table.  I 
think I have any typos corrected below.

Oracle support says it is a bug, no patch available yet, and the work around is 
to set _optimizer_use_feedback=false.  Now I need to figure out the 
ramifications of doing that.


> SELECT  'Data4',
>         wdata.created,
>         wdata.value2
>    FROM wdata, wbedata
> WHERE wbedata.my_number = '888888'
>     AND (wdata.created <= (select trim(value_string) from other.parm_value 
> where job_type = 'D TEST' and parm_type = 'B END') || '-23.59.59.999999'
>     AND (wdata.created >= (select trim(value_string) from other.parm_value 
> where job_type = 'D TEST' and parm_type = 'B START') || '-23.59.59.999999'))
>     AND (wdata.created = wbedata.created)
>     AND (wdata.value2 = wbedata.value2)
>     AND (wdata.value3 = wbedata.value3)
>   ORDER BY wdata.created;
Sent from my iPad

> On Oct 31, 2014, at 4:15 AM, Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx> 
> wrote:
> 
> 
> 
> I'm assuming, by the way, that any oddities in the sample code are just the 
> result of trying to edit out the confidential stuff.
> 
> In particular: I've asssumed that there's a unique constraint on (job_type, 
> parm_type) so that the optimizer can "know" that there's only a single 
> possible value; and I've assumed that the subquery is written to supply the 
> column type and hasn't thrown in another obfuscating factor by causing column 
> conversion.
> 
>    
> Regards
> Jonathan Lewis
> http://jonathanlewis.wordpress.com
> @jloracle 
> From: oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] on behalf 
> of Jonathan Lewis [jonathan@xxxxxxxxxxxxxxxxxx]
> Sent: 31 October 2014 08:57
> To: Andrew Kerber; Mark W. Farnham
> Cc: Howard Latham; oracle-l
> Subject: RE: Really strange performance issue
> 
> 
> Looking at the code, I think Sayan's comment is the relevant one.
> 
> Waving my hands and guessing WILDLY - but I suspect I could create a data set 
> where this happens:
> 
> The optimizer is probably handling your subqueries as "unknown constant", 
> which gives you a range scan on unknown values which gives the optimizer a 
> guess of 0.25% - hence the application of cardinality feedback.
> On the first pass the optimizer drives off the created date - and discovers 
> that it does a lot more work than expected (more rows), so on the second pass 
> it reverses the join, which turns out to be a bad idea because the 
> optimizer's estimated cardinality of '88888' (which doesn't get modified by 
> the first pass) is badly wrong and/or the chosen access path back into wdata 
> is much less efficient than expected.
> 
>    
> Regards
> Jonathan Lewis
> http://jonathanlewis.wordpress.com
> @jloracle 
> From: Andrew Kerber [andrew.kerber@xxxxxxxxx]
> Sent: 31 October 2014 01:32
> To: Mark W. Farnham
> Cc: Jonathan Lewis; Howard Latham; oracle-l
> Subject: Re: Really strange performance issue
> 
> Below is a heavily redacted version of the query, all columns names and 
> values changed,as I said it is pretty straightforward,
> 
> SELECT  'Data4',
>         wdata.created,
>         wdata.value2
>    FROM wdata, wbedata
> WHERE wbedata.my_number = '888888'
>     AND (wdata.created <= (select trim(value_string) from other.parm_value 
> where job_type = 'D TEST' and parm_type = 'B END') || '-23.59.59.999999'
>     AND (wdata.created >= (select trim(value_string) from other.parm_value 
> where job_type = 'D TEST' and parm_type = 'B START') || '-23.59.59.999999'))
>     AND (wdata.created = wbedata.created)
>     AND (wdata.value2 = wbedata.value2)
>     AND (wdata.value3 = wbedata.value3)
>   ORDER BY wdata.created;
> 
>> On Thu, Oct 30, 2014 at 6:44 PM, Mark W. Farnham <mwf@xxxxxxxx> wrote:
>> +42
>> 
>> -----Original Message-----
>> From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
>> On Behalf Of Jonathan Lewis
>> Sent: Thursday, October 30, 2014 4:00 PM
>> To: andrew.kerber@xxxxxxxxx; Howard Latham
>> Cc: oracle-l
>> Subject: RE: Really strange performance issue
>> 
>> 
>> 
>> Although we generally expect cardinality feedback to result in better plans
>> it's possible that a change in plan could change the order in which the data
>> driving (e.g.) a scalar subquery is accessed, increasing the number of times
>> a subquery is executed without changing the number of rows returned in the
>> rowsource. If by "embedded select" you actually mean a scalar subquery it's
>> possible that the main query does look more efficient to the optimizer, but
>> the scalar subquery runs far more time.  Easy to detect if you enable
>> rowsource execution statistics (e.g. add hint gather_plan_statistics) and
>> use the 'allstats last' format option with dbms_xplan.display_cursor().
>> 
>> 
>> 
>> 
>> 
>> Regards
>> Jonathan Lewis
>> http://jonathanlewis.wordpress.com
>> @jloracle
>> 
>> ________________________________________
>> From: oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] on
>> behalf of Andrew Kerber [andrew.kerber@xxxxxxxxx]
>> Sent: 30 October 2014 14:51
>> To: Howard Latham
>> Cc: oracle-l
>> Subject: Re: Really strange performance issue
>> 
>> I'll have to see if I can remove identifying information, but there is
>> really nothing special about it, basically a two table join with a couple of
>> embedded selects to get a date range.  The plan is the same in both cases.
>> 
>> Sent from my iPad
>> 
>> > On Oct 30, 2014, at 9:42 AM, Howard Latham <howard.latham@xxxxxxxxx>
>> wrote:
>> >
>> > Any chance of seeing the Query please?
>> --
>> http://www.freelists.org/webpage/oracle-l
>> 
>> 
>> --
>> http://www.freelists.org/webpage/oracle-l
> 
> 
> 
> -- 
> Andrew W. Kerber
> 
> 'If at first you dont succeed, dont take up skydiving.'

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